A photo-activated process cascaded electrocatalysis for the highly efficient CO2reduction over a core-shell ZIF-8?Co/C

Article abstract of DOI:10.1039/d0ta04595k

Light irradiation can affect electronic properties of catalysts and the introduction of appropriate light into electrocatalysts may have a significant impact on the electrocatalytic process; however, this has not been fully studied. Herein, we propose a photo-activated process cascaded electrocatalysis for CO2 reduction to produce syngas over a core-shell ZIF-8?Co/C catalyst. Under light irradiation, the onset potential and overpotential of ZIF-8?Co/C positively shift by 40 and 200 mV, and the syngas production is enhanced 5.2-fold at a bias potential of -0.9 V vs. RHE. It is noteworthy that the electric energy efficiency is enhanced by 30%. Deducting syngas generated by electricity, the solar-to-syngas conversion efficiency (joule to joule) is as high as 5.38%, which outperforms reported photoelectrochemical systems. These devices also relatively maintain high efficiency in neutral pH aqueous solution. Dedicated experiments and in situ transient photovoltage studies demonstrate that the cascaded photo-activation of CO2 and H+ in electrocatalysis accounts for the outstanding catalytic performance. This journal is

Full text of DOI:10.1039/d0ta04595k

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Photo-activated process cascaded electrocatalysis for highly
efficient CO2 reduction over core-shell ZIF-8@Co/C
Received 00th January 20xx,
Accepted 00th January 20xx
Jian-Xia Gu,a Xue Zhao,a Yue Sun,b Jie Zhou,a Chun-Yi Sun,*a Xin-Long Wang,*a Zhen-Hui Kang*b and
Zhong-Min Sua
DOI: 10.1039/x0xx00000x
Light irradiation could affect electronic properties of catalysts and the introduction of appropriate light into the
electrocatalysts may have a significant impact on the electrocatalytic process, but it has not been fully studied. Herein, we
propose a strategy of photo-activated process cascaded electrocatalysis for CO2 reduction to produce syngas over core-
shell ZIF-8@Co/C catalyst. Under light-irradiation, onset potential and overpotential of ZIF-8@Co/C positively shifts by 40
and 200 mV, and syngas production enhances by 5.2-fold at bias potential of -0.9 V vs. RHE. It is noteworthy that the
electric energy efficiency promotes by 30%. Deducting syngas generated by electricity, the solar-to-syngas conversion
efficiency (Joule to Joule) is up to 5.38% which outperforms reported photoelectrochemical systems. The devices also keep
relatively high efficiency in neutral pH aqueous solution. The dedicated experiments and in situ transient photovoltage
illuminate the cascaded photo-activation of CO2 and H+ in electrocatalysis accounts for the outstanding catalytic
performance.
RHE which is quite higher than that of methane, hydrocarbons and
alcohols.11 A substantial amount of additional potential is required
1. Introduction
Excessive utilization of limited fossil fuels accompanied by heavy
emission of CO2 has aggravated global energy crisis and
environmental burden.1-3 The electrochemical reduction of CO2 to
useful chemical stocks or fuels, such as syngas, provides a promising
technology to realize a circular carbon-neutral cycle.4-6 Over the
past several years, substantial progress has been made in
understanding the transformations of CO2 since it is kinetically
sluggish and intricate reduction process with a high energy barrier.
Particularly, the activation of CO2 molecule which requires a
reduction potential of −1.9 V (versus standard hydrogen electrode
to drive CO2RR. Therefore,
a
deeper understanding of
electrocatalysts-CO2 activation and the associated kinetics are still
necessary, which will lead to more detailed strategies to design
CO2RR catalysts with low overpotential. Note that, light irradiation
possibly affects electronic properties of catalysts, such as Fermi
level, electron transfer, charge distribution and desorption energy
of intermediates, and even influences catalytic pathways.12
Unfortunately, it is difficult to control these changes induced by
light to orientated affect the specific step of catalyst in CO2
reduction. When a light irradiation process is cascaded directly with
the key electrocatalytic step, it may substantially reduce the
overpotential and reduce the cost of electric energy. However,
there have been few studies investigated the influence of light on
CO2 electroreduction.
*−
[SHE]) to reduce linear CO2 to bent radical anion CO2 has the
highest energy barrier.7, 8 Although multi-proton-assisted electron-
transfer processes can drive the relatively low activation energy
barrier, CO2 reduction reaction (CO2RR) remains thermodynamically
10
and kinetically difficult.9,
For example, the thermodynamical
Recently, metal-organic frameworks (MOFs) have been
considered as great potential catalytic materials in the field of
electrocatalysis because of their well-defined composition, large
equilibrium potential for CO2 reduction to CO is up to -0.11 V vs.
specific surface area, permanent porosity and various catalytic
14
centers .13,
With classical MOF-based material as a catalyst,
a. National & Local United Engineering Laboratory for Power Batteries, Department
of Chemistry, Northeast Normal University, Changchun, Jilin, China. E-mail:
suncy009@nenu.edu.cn; wangxl824@nenu.edu.cn.
herein, we report a strategy of photo-activated process cascaded
electrocatalysis for highly efficient CO2RR to produce syngas. Core-
shell ZIF-8@Co/C catalyst with hierarchical porous structure is
designed based on the following considerations: i) Co/C shell
guarantees excellent light absorption and electrical conductivity; ii)
mesopores on the shell facilitate the diffusion of substrates; iii) ZIF-
8 core acts as CO2 electrocatalyst for its low cost (0.8$/g) and
b. Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute
of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou,
Jiangsu, China. E-mail: zhkang@suda.edu.cn.
bInstitute of Advanced Materials, Northeast Normal University, 5268 Renmin
Street, Changchun 130024, P.R. China.
Electronic Supplementary Information (ESI) available: Experiment details,
characterization and supporting electrochemical and photo-activated
electrochemical performances. See DOI: 10.1039/x0xx00000x
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convenient synthesis.15 ZIF-8@Co/C with different shell thicknesses
is synthesized by partial pyrolysis of the corresponding ZIF-8@ZIF-
67. Without conductive agents, ZIF-8@Co/C exhibits outstanding
electrocatalytic performance with Faraday efficiency of syngas
(mixture of CO and H2) generation nearly 100% at -1.2 V vs. RHE.
Under light irradiation, the onset potential positively shifts by 40
mV, current density increases by 1.6-fold and syngas production
elevates by 5.2-fold at bias potential of -0.9 V vs. RHE together with
a decreased overpotential of 200 mV for a similar yield value and a
promotion of 30% on electric energy efficiency. Significantly,
conversion efficiency (Joule to Joule) of solar energy to syngas
(deducting the syngas generated by electricity) is up to 5.38%,
which outperforms the reported photoelectrochemical systems.
Combined with dedicated electrochemical test and in situ transient
photovoalt (TPV), we clearly show that ZIF-8 is more likely to
produce CO while Co/C mainly generates H2 and acts as the main
source of photo-induced electrons. The generated photo-induced
electrons can orientatedly transfer from Co/C to ZIF-8 to trigger CO2
activation because of the opposite charge between shell and core
of ZIF-8@Co/C catalyst. This photo-activated process cascaded
electrocatalysis significantly reduces the overpotential of
electrochemical CO2 reduction (See Scheme 1) and leads to high
catalytic performance.
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DOI: 10.1039/D0TA04595K
Scheme
1
Diagram of the photo-activated process cascaded
electrocatalytic pathway over ZIF-8@Co/C catalyst. (E: optimal
applied potential; Eonset: onset potential; EE: electric energy
efficiency; η: solar-to-syngas conversion efficiency).
furnace. Then, the powder was heated to 550 °C and kept for 8 min
under flowing N2. When the temperature cooled to room
temperature, the ZIF-8@Co/C-x (x=1 to 5) was fabricated. The
pyrolysis process of ZIF-8 and ZIF-67 is similar to the above samples.
Electrochemical measurements
The light-irradiation cascaded electrochemical and electrochemical
reduction of CO2 were executed in an H-type cell (Nafion 117
membrane) with a typical three-electrode system. Pt sheet, Ag/AgCl
electrode (filled with 3 M KCl) and glassy carbon electrode (GCE) or
carbon paper were acted as counter electrode, reference electrode
and working electrode, respectively.
2. Experimental section
Materials
Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and 2-methylimidazole (2-
MeIm) were purchased from the Shanghai Census Bio-tech Co., Ltd..
Cobalt chloride hexahydrate (CoCl2·6H2O) and methanol were
bought from the Fuchen chemical reagent field in the city of Tianjin
and Beijing Chemical Works, respectively.
The working electrode of light-irradiation cascaded
electrochemistry was prepared as following: catalyst (5 mg) was
first dispersed in Nafion solution (1.0 mL 0.5%) and ultra-sonicated
for 1 h to generate a homogeneous ink. Then, catalyst (20 µL) was
dropped onto two sides of carbon paper (0.5*1.0 cm2) and dried
under air atmosphere. K2SO4 aqueous solution (0.25 M) was used as
the electrolyte according to the reported literature. Before the
tests, the electrolyte was purged with CO2 (or Ar) for 30 min (pH =
4.65, CO2- saturated K2SO4 solution). During measurements,
working electrodes were irradiated by a xenon lamp equipped with
an AM 1.5 filter. According to the Nernst equation E (RHE) = E
(Ag/AgCl) + 0.197 + 0.0591 × pH, all the potentials were calibrated
to a reversible hydrogen electrode (RHE). The products of CO2
electrochemical reduction were detected through gas
chromatography (GC) (GC2014, Shimadzu, Japan) equipped with FID
and TCD detectors. The process of electrochemical measurement is
similar to the light-irradiation cascaded electrochemical test in
addition to changing the working electrode and giving no
illumination. In this system, GCE (diameter, 3 mm) coated with ZIF-
Preparation of the serial of ZIF-8@ZIF-67-x16 and ZIF-8@Co/C-x
(x=1 to 5)
ZIF-8 (80 mg) seed was dispersed in 10 mL of methanol and ultra-
sonicated for 30 min. Then, a methanolic solution of CoCl2·6H2O
(177 mg, 3 mL) and a methanolic solution of 2-MeIm (895 mg, 3 mL)
were slowly dripped into the above mixture. Subsequently, the
mixture was transferred into an autoclave and kept at 100 °C for 12
h. The resulted sample was named as ZIF-8@ZIF-67-4 and was
collected by centrifugation after cooling, then washed several times
with methanol and dried at 80 °C. The other ZIF-8@ZIF-67-x (x=1, 2,
3, 5, x is defined as the number of samples) with different
thicknesses of ZIF-67 shell were prepared by adjusting the amount
of CoCl2·6H2O, 2-MeIm and methanol, and the specific amount was
shown in the Table S1. The synthetic method is similar to that of
ZIF-8@ZIF-67-4.
ZIF-8@Co/C-x (x=1 to 5) was obtained by partial pyrolysis of
corresponding ZIF-8@ZIF-67-x (x=1 to 5). The realization of partial
pyrolysis strategy is attributed to the difference of pyrolysis
temperature of ZIF-8 and ZIF-67.17 To be specific, the powder of ZIF-
8@ZIF-67-x was put into a ceramic boat and transferred into a tube
8@Co/C (5 mg/mL,
8 µL) is used as a working electrode.
Electrochemical impedance spectroscopy (EIS) measurement was
carried out on CHI760 in a frequency range from 100 kHz to 0.01 Hz
at a potential of -0.9 V (vs. RHE).
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In situ transient photovoltage (TPV) measurements
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DOI: 10.1039/D0TA04595K
Under room temperature, the transient photovoltage
measurements were carried on films of samples deposited on
indium-tin oxide (ITO) glass substrates (1 cm × 1 cm). The films of
samples were prepared by dispersing samples (5 mg) into 1.0 ml
aqueous (700 μL water, 250 μL ethanol and 50 μL of Nafion
solution), then dropped sample aqueous on ITO slides and dried in
air. The ITO glass modified with samples as the working electrodes
and Pt wire as the counter electrodes were wetted with
acetonitrile/H2O aqueous (v/v=4:0.5). The samples were excited by
a laser radiation pulse (wavelength 355 nm, pulse width 5 ns) from
a third-harmonic Nd: YAG laser (Polaris II, New Wave Research,
Inc.).
3. Results and discussions
Fig. 2 Structure characterization of ZIF-8@ZIF-67-4 and ZIF-8@Co/C-
4. (a) N2 adsorption–desorption curves of ZIF-8@ZIF-67-4 and ZIF-
8@Co/C-4. (b) Pore-size distribution plots of ZIF-8@ZIF-67-4 and
ZIF-8@Co/C-4. (c) Co 2p and Zn 2p XPS spectra of ZIF-8@Co/C-4. (d)
UV-Vis spectra of ZIF-8@ZIF-67-4 and ZIF-8@Co/C-4.
Materials characterization
A series of core-shell ZIF-8@ZIF-67-x (x is defined as the number of
samples from 1 to 5) with different thicknesses of shell were
prepared according to reported methods16 and through partial
pyrolysis of ZIF-8@ZIF-67-x, ZIF-8@Co/C-x were obtained (Fig. 1a).
The PXRD (powder X-ray diffraction) pattern of ZIF-8@ZIF-67-x is
aligned well with that of bare ZIF-8 or ZIF-67 as the pattern of ZIF-8
of Co/C shell. The molar ratios of Co2+/Zn2+ are probed by
inductively coupled plasma mass spectrometry (ICP-MS) and the
thicknesses of Co/C in ZIF-8@Co/C-x are obtained through line
scans (Fig. 1d and Fig. S3b), which is depicted in Table S2.
18
and ZIF-67 is similar (Fig. S1a).16, After pyrolysis, ZIF-8@Co/C-x
maintains pristine PXRD pattern of ZIF-8@ZIF-67-x due to intact
structure of core ZIF-8 (Inset a of Fig. S1b), but there is a new weak
peak at about 44.9° (Fig. S1b) which can be ascribed to cobalt
nanoparticles (CoNPs, JCPDS No.15-0806) resulting from the
decomposition of ZIF-67 (Inset b of Fig. S1b).19, 20
The TEM images of ZIF-8@Co/C-x (x=1 to 5, Fig. 1b and Fig. S3a)
retain rhombic dodecahedron morphology of ZIF-8@ZIF-67-x (x=1
to 5, Fig. S2a), except the emergence of rough edge. High-resolution
TEM (HRTEM) image of rough edge (Fig. 1c) reveals two types of
lattice fringes with d-spacing of 0.21 and 0.36 nm which belong to
CoNPs (111) and graphitic carbon (002),21, 22respectively, from the
decomposed ZIF-67 shell. The elemental mappings (Fig. 1e and Fig.
S3c) of Zn and Co in ZIF-8@Co/C-x show they separately distribute
For all ZIF-8@ZIF-67-x (x=1-5), the N2 adsorption-desorption
curves (Fig.2a and Fig. S4) belong to type-I isotherm, which
indicates that ZIF-8@ZIF-67-x is a kind of microporous material and
the pore size of them is mainly distributed in the range of 1-2 nm
(Fig. 2b and Fig. S5). The thickness of the shell has little effect on
the N2 adsorption-desorption as well as pore size distribution of the
ZIF-8@ZIF-67-x. In contrast, the N2 adsorption-desorption curves of
ZIF-8@Co/C-x (x=1-5) all belong to type-Ⅳ isotherm with a H3-type
hysteresis loop, indicating the existence of abundant mesopores
(Fig.2a and Fig. S4)17, 24, 25. With the increase of shell thickness in
ZIF-8@Co/C-x, the adsorption capacity of nitrogen decreases
gradually, which is due to the reduction of specific surface area
originating from the transformation of a certain amount of
micropores into mesopores after calcination. It is obvious when the
shell thickness is lower than 50 nm, the pore size distribution of ZIF-
8@Co/C-x (x=1-4) is not affected by the shell thickness, and there
are two kinds of pore sizes locating in 1-2 nm and 3-4 nm. Yet, when
the thickness of the shell is larger than 110 nm (ZIF-8@Co/C-5),
mesopores in the range of 2-3 nm emerges (Fig. 2b and Fig. S5). In a
word, ZIF-8@Co/C-x (x=1-5) exhibits hierarchical pore structure,
which can facilitate the transport of CO2 (aq) or CO2RR relevant
species in solution.26, 27 The specific surface area of ZIF-8@Co/C-x
(x=1-5) reduces by 1 %, 8 %, 30 %, 40% and 80 % in comparison with
ZIF-8@ZIF-67-x (x=1-5), just shown in Table S3. This decrease is
likely due to the increased mesoporous ratio in the ZIF-8@Co/C-x.
The CO2 adsorption capacity of ZIF-8@Co/C-4 is measured to be of
~10.8 cm3/g (0.44 mmol/g) at room temperature and 1.0 atm (Fig.
S6a), suggesting that hierarchical pores offer accessible sites for the
in the centre and edge as that in ZIF-8@ZIF-67-x (Fig. S2b),
suggesting ZIF-8@Co/C-x maintains core-shell structure.16,
gradually increased Co content represents an incremental thickness
23
The
Fig. 1 Preparation and Electron Microscopic Characterization of ZIF-
8@ZIF-67 and ZIF-8@Co/C. (a) Schematic illustration of the
synthesis process of ZIF-8@ZIF-67-x and ZIF-8@Co/C-x (x=1 to 5).
(b) TEM image, (c) HRTEM image, (d) line-scanning spectrum and (e)
elemental mapping of ZIF-8@Co/C-4: Zn (purple) and Co (yellow).
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adsorption or capture of CO2 molecules.26 X-ray photoelectron
spectroscopy (XPS) of ZIF-8@Co/C-4 reveals the predominant
existence of C, N, O, Co and Zn element (Fig. S6b). High-resolution
XPS spectra at Co 2p and Zn 2p in Fig. 2c suggest that Co0 and Zn2+
species are in the ZIF-8@Co/C-4, which could corroborate the
existence of CoNPs28-30 and ZIF-831. The presence of CoNPs is further
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DOI: 10.1039/D0TA04595K
witnessed by magnetism of samples in
a video (Supporting
Material). The diffuse reflectance ultraviolet-visible (UV-Vis)
spectrum was used to investigate the optical absorption ability of
catalysts. In contrast to the two absorption bands in ZIF-8@ZIF-67-4
(200-350 and 450-650 nm), the UV-Vis spectrum of ZIF-8@Co/C-4
expands the absorption to the whole visible light (400-800 nm) with
high intensity (Fig. 2d). Considering ZIF-8 only absorbs the light in
the ultraviolet region (Fig. S6c), therefore, the visible-light
absorption of ZIF-8@Co/C-x (x=1 to 5, Fig. S6c) mainly originates
from Co/C shell. This excellent light absorption ability of ZIF-
8@Co/C may provide a prerequisite for the response to light
irradiation. The graphitic degree of ZIF-8 and ZIF-8@Co/C-x (x=1 to
5) was investigated by Raman spectroscopy (Fig. S6d). As seen, ZIF-
8@Co/C-x (x=1 to 5) shows an obvious peak at about 1500 cm-1 and
a weak peak near 1300 cm-1 which is ascribed to G band and D band
of carbon materials, respectively, and indicates a high degree of
graphitization in samples.32 Such structure may lead to high
conductivity which conforms to the design requirements for
electrocatalysts. But there are no similar peaks in the Raman
spectroscopy of ZIF-8, reflecting that disordered carbon and
graphite carbon results from pyrolytic ZIF-67.
Fig. 3 Electrochemical properties of various catalysts for CO2RR on
GCE. (a) The LSV responses of ZIF-8, ZIF-8@Co/C-4 and Co/C in 0.25
M Ar- and CO2-saturated K2SO4. (b) Faradaic efficiency of CO and H2
and the corresponding proportions of syngas (CO/H2) on the ZIF-
8@Co/C-x (x=1 to 5) at -1.2 V vs. RHE.
to the thickness of ZIF-8@Co/C shell while the FE of H2 is just
reverse (Fig. S9b). The reason may be that promoted thickness of
Co/C shell limits the diffusion of CO2 to centre ZIF-8, but provides
more active sites for H2 production. These results testify that molar
ratios of CO/H2 are definitely related to composition in catalyst and
can be tuned in a large range from 3:1 to 1:5 (Fig. 3b). It is
noteworthy that the FE(CO+H2) of these catalysts are all close to
100%. Compared with the performance of reported electrocatalysts
(summarized in Table S4) toward syngas generation, the wide range
of CO/H2 ratios achieved on the ZIF-8@Co/C-x is outstanding, and is
beneficial to satisfy the demands for various industrial applications,
for example, H2: CO = 0.3−1 is used for syngas fermentation and H2:
CO ≥ 2 is the important raw materials for methanol synthesis or
Fischer-Tropsch reactors.35-38
The electrocatalytic CO2 reduction
The electrochemical performance of ZIF-8@Co/C was firstly
evaluated on the glass carbon electrode (GCE) in K2SO4 (0.25M,
pH=4.65) aqueous solution following the reported model.33 As a
control, the performance of bare ZIF-8 and Co/C was also studied.
Linear sweep voltammetry (LSV) curves at 10 mV s-1 from -0.3 to -
1.3 V vs. RHE were first tested to evaluate electrocatalytic
performance (Fig. 3a and Fig. S7). The current density of these
samples in Ar can be ascribed to hydrogen evolution reaction
(HER).34 ZIF-8@Co/C-x (x=1 to 5) and ZIF-8 exhibit a higher current
density in CO2 than that in Ar, while Co/C doesn’t present any
obvious difference, which implies the active site for CO2RR in ZIF-
8@Co/C may be ZIF-8. Comprehensive product analysis by gas
chromatography (GC) reveals that CO and H2 are the main gas
products. And the isotopic experiment was performed by using
13CO2 labeling, the 13CO signal (m/z = 29) demonstrates the carbon
source of CO indeed derives from the CO2 used (Fig. S8). Based on
the above, we preliminary conclude that ZIF-8 is the main active site
of CO generation while Co/C is the centre of H2 production. In order
to further illustrate this conclusion, the yield of CO and H2 was
detected on ZIF-8 and Co/C at -1.2 V vs.RHE. As shown in Fig. S14,
H2 generated by Co/C on GCE is 53.1 times that of ZIF-8, while CO
generated by ZIF-8 is 18.7 times that of Co/C, indicating the truth of
the above conclusion.
ZIF-8@Co/C-1 is taken as a representation of ZIF-8@Co/C-x for
investigating long-term stability. The electrocatalytic stability was
evaluated by continuous electrolysis of CO2 at -1.2 V vs. RHE for 8 h
(the analysis of gaseous product by GC every 1.5 h), during which no
prominent deactivation of current density and stable FE for CO and
H2 were observed in Fig. S10. Such durability means that ZIF-
8@Co/C-x serves as promising electrocatalysts for persistently
producing a wide proportion of syngas during CO2 reduction
process.
Light-irradiation cascaded electrochemical CO2 reduction.
To evaluate the influence of light, Xenon lamp with AM1.5 filter was
used. For the inconvenience of light irradiation, the catalyst was
dispersed on carbon paper instead of GCE in the light-irradiation
cascaded electrocatalysis. First, take ZIF-8@Co/C-4 as an example,
LSV curves were tested under dark and light conditions respectively
to evaluate the electrocatalytic properties for CO2 reduction on
carbon paper. As a control, the LSV curves of pure ZIF-8 and Co/C
were also detected. The current density of ZIF-8@Co/C-4 on carbon
paper is all higher in CO2 than that in Ar both in darkness (Fig.4a)
and light irradiation (Fig. S11a). And the trend of LSV curves for ZIF-
8 (Fig. S11b) is similar to that of ZIF-8@Co/C-4, suggesting that ZIF-8
and ZIF-8@Co/C-4 have catalytic activity for electrochemical CO2
reduction. As for Co/C (Fig. S11c), the current density in CO2 is
It is found at an optimized applied potential of -1.2 V vs. RHE,
the Faradaic efficiency (FE) of CO (Fig. S9a) is negatively correlated
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detected under dark and light irradiation conditions. The
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DOI: 10.1039/D0TA04595K
production rate of CO (Fig. 4c) declines with the increase of Co/C
thickness while that of H2 (Fig. 4d) rises and the trend is likely to
that in the electrocatalysis on GCE (Fig. S9). Besides, H2 generated
by Co/C on carbon paper (CP) is 4.4 times that of ZIF-8, while CO
generated by ZIF-8 is 5.1 times that of Co/C (Fig. S14) at -0.9 V
vs.RHE, which further indicates that the generation of CO is still
mainly on core ZIF-8 while H2 is on shell and light irradiation doesn’t
change catalytic center. In comparison with the dark, the yield of
syngas on ZIF-8@Co/C-x (x=1 to 5) at -0.9 V vs. RHE enhances by
3.8-5.2 folds under light irradiation (Table S5). The improvement of
ZIF-8@Co/C-4 in the amount of syngas exceeds the other four
samples and the yield of CO and H2 reaches 17 and 926 mmol g-1 h-
1, respectively, under light irradiation. The achievement of similar
yield in the dark needs a potential of -1.1 V vs. RHE (Fig. 4b). That is
to say, by light irradiation, the overpotential to reach a similar yield
value could be reduced by 200 mV. The decreased overpotential
further verifies that the cascaded light-irradiation process serves as
the role of pre-activation to promote catalytic reactivity. Moreover,
the largely increased yield also means that it is possible to improve
the electric energy efficiency (EE) of electrochemical process.
According to the calculation formula (Supporting Information) of EE,
an EE of 51.0% is obtained for ZIF-8@Co/C-4 by light irradiation,
which increases by ~30% comparing with that in the dark (39.0%).
Solar energy utilization efficiency in this system is evaluated
from the conversion efficiency of Joules of sunlight (in) to Joules of
utilizable fuel (out) which is often employed in photocatalysis or
photoelectrochemistrysis.39, 40 The solar-to-syngas (Joules to Joules)
energy conversion efficiency of ZIF-8@Co/C-x (x=1 to 5) is
calculated as following:39
Fig. 4 Light-irradiation cascaded electrochemical performance of
CO2 reduction on the carbon paper. (a) LSV curves of ZIF-8@Co/C-4
in Ar-saturated 0.25 M K2SO4 in the dark (black line) and CO2-
saturated 0.25 M K2SO4 under illumination (orange line)/dark (gray
line). The inset shows the change of onset potential (1 mA cm-2)
after illumination in CO2-saturated 0.25 M K2SO4. (b) The curves of
production rates for CO and H2 on ZIF-8@Co/C-4 at different
potentials under illumination (orange)/dark (gray). (c, d) The
production rates of (c) CO and (d) H2 on the ZIF-8@Co/C-x (x=1 to 5)
in the dark (gray) and illumination (orange) at -0.9 V vs. RHE.
almost identical comparing to that in Ar, meaning the active site for
CO2RR in ZIF-8@Co/C may be ZIF-8. The trend of LSV curves above is
all in agreement with that on GCE, indicating the carbon paper
doesn’t affect the CO2RR activity of catalysts. With light irradiation,
the LSVs of ZIF-8@Co/C-x on carbon paper are obtained and show
much enhanced electrochemical activity in contrast to in the dark
under similar condition, mainly reflected in the increased current
density and the positive shift of onset potential (Fig. 4a and Fig.
S12). The current density of ZIF-8@Co/C-x (x=1 to 5) enhances by
1.1 to 1.6-folds at -1.3 V vs. RHE in comparison with that in the
dark. The onset potential of ZIF-8@Co/C-x (x=1 to 5) positively shifts
by 20 to 40 mV. Among them, ZIF-8@Co/C-4 presents the best
performance with a 1.6-fold increase in current intensity and 40 mV
shifts of onset potential. The significantly reduced onset potential in
the electrocatalysis suggests the introduction of light may be
assistant to reduce the energy barrier of CO2 activation. The best
performance of ZIF-8@Co/C-4 may ascribe to the appropriate
thickness of shell in light absorption and electron transport.
(n1 -n2)CO cHC0O (n1 -n2)H cHH0
2
2
 
Psolar t
In the equation, n1 is the molar of CO and H2 in the light-irradiation
cascaded electrochemistry, while n2 is the molar of CO and H2 in the
electrochemistry. Psolar is the input power of the xenon lamp (450
mW cm-2, for 0.25 cm2 illuminated area). ΔcH0 is the standard heat
of combustion of H2 and CO (298.15 K).
The efficiency over ZIF-8@Co/C-x (x=1 to 5) is presented in
Table S5. It is found the maximal conversion efficiency is 5.38% over
ZIF-8@Co/C-4, which exceeds the reported photoelectrochemical
systems (Table S6). At this time, the syngas ratio (H2/CO) is 54/1 >
50, which can be employed in power generation including gas
turbine and fuel cell as well as refinery hydrotreating.38 ZIF-
8@Co/C-4 is taken as a representation of ZIF-8@Co/C-x to study
photostability (Fig. S15a). Negligible degradation of the current
density and stable molar ratio of H2/CO are observed on ZIF-
8@Co/C-4 catalyst after a continuous test for 8 h at an applied
potential of -0.9 V vs. RHE (the analysis of gaseous products by GC
every 2 h), suggesting that the as-prepared ZIF-8@Co/C is a
durability for electrochemical reduction CO2 by light irradiation. To
further prove the chemical stability of catalyst, XRD of ZIF-8@Co/C-
4 was tested after reaction for 8h (Fig. S15b). The main peaks of
XRD after reaction for 8h is similar to that before the reaction,
revealing its robust chemical stability during the catalysis. After
Based on the above LSVs, the effect of light irradiation on yield
of products was also investigated. The potentials were firstly
optimized by measuring the yield at different potentials under
illumination and -0.9 V vs. RHE was determined to be the best
potential for the highest yield of CO (Fig. S13). At this potential, the
production rates of CO and H2 over ZIF-8@Co/C-x (x = 1 to 5) were
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catalytic reaction for 8h, the XPS of Co2p (Fig. S15c) shows that Co0
and Co-O species in Co/C shell, which is consistent with Co before
reaction and also proves excellent chemical stability of catalyst.
Reproducibility of a catalyst is a key element in electrochemical
reduction of CO2. Thus, take optimal sample ZIF-8@Co/C-4 for
example, the catalytic experiments with six batches of the same
catalyst were carried out to study the reproducibility under
illumination and the yield of products (CO and H2) at each batch
was obtained by gas chromatography. Just shown in Fig. S16, the
yield of CO and H2 is almost identical. And the relative standard
deviations (RSDs) are calculated to be 4.9 % for CO and 2.0 % for H2
from Fig. S16, suggesting the catalyst in this system shows favorable
reproducibility for CO2 electrochemical reduction.
Comparing with high-energy light exposure (with AM 1.5 filter),
the production rates of CO and H2 decrease under visible-light
illumination (with 420 nm-cutoff filter), which can be attributed to a
component in ZIF-8@Co/C-4 with the wide bandgap led to a limited
excitation by visible light. The yield of CO and H2 on ZIF-8@Co/C-4
still remains 3.2 mmol g-1 h-1 and 343.5 mmol g-1 h-1 under visible-
light illumination (Fig. S17a), respectively, indicating ZIF-8@Co/C-4
can maintain a satisfying catalytic effect under visible light. It is a
favorable signal to make use of sunlight as visible light accounts for
~50% of sunlight. 42 When the light density reduces from 450 to 225
mW cm-1 (Fig. S17b), the yield of CO and H2 just reaches a half
indicates there is no photo-induced side-products in the process of
reaction. It is found that, in the neutral electrolyte solution
(pH=7.12, Fig. S17c), this system is still highly active but relative to
the faintly acid solution (pH=4.65), the yield of CO and H2 descends
by 36% (10.7 mmol g-1 h-1) and 57% (398.2 mmol g-1 h-1). Such a
result can be due to the decreased amount of H+ in the neutral
electrolyte solution. The high yield in the neutral electrolyte
solution reflects that ZIF-8@Co/C-4 can be applied to
electrochemical reduction of CO2 under a wide range of conditions.
View Article Online
DOI: 10.1039/D0TA04595K
Fig. 5 Mechanism investigation of photo-activated electrocatalytic
CO2 reduction. (a-c) TPV responses of using ZIF-8 (a), ZIF-8@Co/C
(b) and Co/C (c) as electrode. (d) The scheme of photo-activated
process of CO2 and H+ and the process of electron transmission in
the catalyst under light irradiation.
ZIF-8@Co/C in N2 atmosphere (black curve, Fig. 5b) with that of
Co/C (black curve, Fig. 5c) in the same condition, we could find 20%
enhancement in ZIF-8@Co/C, indicating the core-shell structure
facilitate e-/h+ charge on the surface of core and shell. The TPV
responses for Co/C are positive (red and black curve) (Fig. 5c) while
for ZIF-8, they are negative (red and black curve) (Fig. 5a). This
43, 44
nature of opposite charge in core and shell
provides an
appealing force for the directional flow of photogenerated
electrons from Co/C to ZIF-8. This will not only facilitate photo-
induced charge separation but accumulate electrons in catalytic
sites.
For comprehensively understanding the role of photo-
irradiation in electrochemical reduction of CO2, the electrochemical
impedance spectroscopies (EIS) were measured to investigate the
effect of light irradiation on the charge transfer resistance (Rct).
Nyquist plots of ZIF-8@Co/C-x (x = 1 to 5) were tested in the dark
(Fig. S18a) and light irradiation (Fig. S18b). The results illuminate Rct
of each catalyst significantly reduces by light stimulation, of
particular importance is the Rct of ZIF-8@Co/C-4 which sharply
decreases from 103 Ω (in the dark) to 39 Ω (under light irradiation).
Such a decrease unambiguously illustrates the influence of the
irradiation on relieving the charge transfer resistance and
enhancing the efficiency of photo-induced charge separation.
As witnessed by the lowered onset potential and overpotential
in the light-irradiation cascaded electrocatalysis, the orientatedly
accumulated electrons in ZIF-8 probably participate in the
activation of CO2. Based on the above experimental results and
speculative analysis, we conjecture the influence of light irradiation
on CO2 reduction, as shown in Fig. 5d. With the assistance of
illumination, photo-induced electrons are generated on Co/C shell
and can activate H+ to form H*, making hydrogen production
relatively easy. Benefiting from the force from the opposite charge
in core and shell, these electrons also conveniently transfer to ZIF-8
core and trigger the activation of CO2 adsorbed by sp2 C atoms of
ZIF-8.33, 45 The above-mentioned process is called photo-activated
Photo-activated electrocatalytic reduction mechanism
To reveal the rooted reason behind the high reactivity, in situ
transient photovoalt (TPV) test using different materials as
electrodes after adding CO2 or N2 was studied firstly. Using bare ZIF-
8 as an electrode (Fig. 5a), comparing with the TPV signal in N2
atmosphere (black curve), the intensity of it in CO2 atmosphere (red
curve) decreases by 61% together with severe trailing, indicating
CO2 may interact with ZIF-8 via multi-step or slow reaction. In
contrast, replacing ZIF-8 with Co/C (Fig. 5c), no appreciable
difference of TPV signal is found under CO2 (red curve) and N2
atmosphere (black curve). These results indicate CO2 tends to
interact with ZIF-8 rather than Co/C. With ZIF-8@Co/C as an
electrode (Fig. 5b), the TPV curve in CO2 atmosphere (red curve)
possesses similar slow decay as that of bare ZIF-8 but the intensity
undergoes 3-fold enhancement than that of ZIF-8 (red curve, Fig.
5a). These data show that under irradiation, ZIF-8 core in the
composite serves as the CO2 reductive centre, while the Co/C shell
mainly takes as the role to absorb light and generates electrons to
activate CO2 and H+. These inferences basically agree with the
conclusions in experiments. When comparing the TPV intensity of
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process which plays a key role in decreasing energy barrier for CO2
reduction to prepare syngas.
Center of Suzhou Nano Science & Technology, the Priority Academic
Program Development of Jiangsu HigherDEOdI:u1c0a.1t0io3n9/DIn0sTtAit0u4t5io95nKs
(PAPD), and the 111 Project.
View Article Online
Compared with the traditional photo-electrocatalysts,46-48 this
photo-activated electrocatalysis integrates the photo-collecting
centre and the electrocatalytic site so that the electrons generated
by light can directly participate in the electrocatalytic process,
rather than converting it into the driving voltage of the external
circuit (for traditional photoelectron-catalyst). This change greatly
improves the efficiency of photogenerated electron utilization,
which may also be the fundamental reason for the high Joule-to-
Joule conversion efficiency (5.38% at -0.9 V vs. RHE) of solar energy
in our system.
Notes and references
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cascaded electrocatalysis for efficient CO2 reduction and
synthesized a kind of photo-activated electrocatalyst ZIF-8@Co/C
by partial pyrolysis of core-shell porous ZIF-8@ZIF-67. The core-
shell and hierarchical porous structure of ZIF-8@Co/C lead to
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vs. RHE in absence of conductive agents. Motivated by light
irradiation, the onset potential and overpotential lower by 40 and
200 mV accompanying with 5.2-fold enhancement in syngas
production at a bias potential of -0.9 V vs. RHE. These lead to a 30%
promotion on electric energy efficiency. More significantly, high
solar-to-syngas (Joule-to-Joule) conversion efficiency is achieved, up
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exceeds
the
value
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This work is supported by the National Natural Science Foundation
of China (21971032, 21671034, 21471027, 51725204, 21771132,
21471106, 51972216), National MCF Energy R&D Program
(2018YFE0306105), the Natural Science Foundation of Jiangsu
Province (BK20190041, BK20190828), Guangdong Province Key
Area R&D Program (2019B010933001), Collaborative Innovation
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Journal of Materials Chemistry A
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DOI: 10.1039/D0TA04595K
Graphical abstract
A photo-activated process is cascaded to electrocatalytic pathway for reducing CO2 to prepare
syngas over core-shell ZIF-8@Co/C, exhibiting excellent electrochemical performance and
achieving high Joule-to-Joule conversion efficiency of 5.38%.